The present invention relates to an open-cell porous material, a method for preparing the same and a mold for pressure-cast-molding a ceramic article which comprises, as a filter medium, the open-cell porous material.
As means for preparing open-cell porous materials used as, for instance, filter mediums, air diffuser materials and mold materials, there have been proposed various methods such as sintering of metallic powder; sintering of thermoplastic resin powder; sintering of inorganic powder; cure of cements through hydration; pressing or stamping of mixtures of thermosetting resins and fillers; a method comprising curing a resin solution containing a pore-forming agent and then removing the pore-forming agent through dissolution-extraction or evaporation; a method which makes use of a blowing agent; and a method comprising curing and polymerizing a W/O type emulsion such as water-containing polyester resin and then evaporating the water.
However, various problems arise when open-cell porous materials are prepared according to these methods. For instance, these methods are substantially limited in shapes and sizes of the resulting products, the preparation thereof often requires a heat-treatment at a high temperature and a high-pressure pressing or they suffer from a problem concerning molding in that the production processes are very complicated.
Moreover, it is very difficult, for these methods, to control the pore size of the porous material which is the most important factor when the resulting porous materials are used as filter mediums and air diffusion materials.
As a method which permits the solution of these problems and the production of a large open-cell porous material having complicated shapes and a desired pore size with a high dimensional accuracy, there has been known a method which comprises stirring a mixture of an epoxy resin, a curing agent, a filler and water to give an emulsion slurry; and then curing the resulting water-containing slurry per se to form an open-cell porous material in which the portion occupied by the water present in the emulsion slurry subsequently form pores. For instance, Japanese Patent Publication for Opposition Purpose (hereinafter referred to as "J.P. KOKOKU") No. Sho 53-2464 accomplishes desired purposes by preparing an O/W type emulsion slurry comprising a glycidyl epoxy resin, a polymerizable fatty acid polyamide curing agent, a filler and water; casting the slurry in a water-impermeable mold to cure the slurry in the water-containing state; and then removing the water. This method permits the production of a large open-cell porous material having a complicated shape with a high dimensional accuracy and can control the pore size by properly selecting the particle size of the filler, the amount of a reactive diluent and the relative amounts of the ingredients used, i.e., the foregoing epoxy resin, curing agent, filler and water. However, the pore size distribution of the open-cell porous material prepared by this method is localized in the region of very low level on the order of not more than 1.5 .mu.m and it is impracticable to use these porous materials in the preparation of filter mediums, air diffusion materials and mold materials.
J.P. KOKOKU No. Sho 62-26657 discloses a method for preparing an open-cell porous material which permits the solution of this problem and which comprises vigorously stirring a mixture comprising a bisphenol type epoxy resin, a curing agent, a filler and water to give an emulsion slurry, the curing agent being a mixture of an amide compound obtained through a reaction of a monomeric fatty acid with an ethyleneamine: H.sub.2 N--(CH.sub.2 --CH.sub.2 --NH).sub.n --H (wherein n is 3 to 5) and a polymeric fatty acid polyamide obtained through a reaction of a polymeric fatty acid with the foregoing ethyleneamine, or a mixed reaction product obtained through a reaction of a mixture comprising the foregoing monomeric fatty acid, polymeric fatty acid and ethyleneamine; casting the emulsion slurry in a water-impermeable mold to cure it in the water-containing state; and then dehydrating the molded slurry. This method allows the production of a large open-cell porous material having a desired average pore size ranging from 0.5 to 10 .mu.m, preferably 0.5 to 5 .mu.m and, in particular, 1.5 to 5 .mu.m and having a complicated shape with a high dimensional accuracy. In addition, Japanese Un-examined Patent Publication (hereinafter referred to as "J.P. KOKAI") No. Sho 63-75044 discloses a method for preparing an open-cell porous material having a pore size ranging from 0.2 to 10 .mu.m which comprises preparing an emulsion slurry from a mixture comprising a glycidyl epoxy resin, a polyamide curing agent and a modified polyamine curing agent and/or an amine curing agent, a filler and water; and then casting the slurry in a water-impermeable mold to cure it in the water-containing state.
These methods permit the control of the pore sizes of the open-cell porous materials. However, the weights of the porous materials are very important when they are used as industrial materials such as filter mediums, air diffusion materials and mold materials. In particular, if these porous materials are used as mold materials for pressure-cast-molding ceramic articles, most of which have complicated shapes as observed in case of sanitary-wares, the parting of a mold is correspondingly complicated and a large number of split molds and cores must be used. In such case, the resulting molded products are often transferred while a part of the mold is held within the molded article and/or hangs down from the exterior thereof during releasing process after completion of the molding. Accordingly, if the weight of the mold is heavy, the molded article is sometimes deformed due to the weight.
Structures of molds for pressure-cast-molding comprise, for instance, a porous dense layer as a superficial filter medium layer (a layer coming in contact with a slurry and capable of absorbing the solvent in the slurry) and a porous coarse layer as a back-up layer on the back-face as those disclosed in J.P. KOKAI No. 63-31710. In this case, the surface layer is in general thin, while the back-up layer is thick and, therefore, the weight of the mold is mainly determined by that of the back-up layer. The back-up layer is obtained by mixing a liquid resin and a filler in a volume ratio of 15.about.50:100 in which interstices formed between individual filler materials serve as pores as disclosed in J.P. KOKAI No. Sho 63-31710 and accordingly, the weight of the mold can be reduced by the use of a light aggregate as the filler material.
In some cases, however, the surface layers occupy the space within the mold at a high rate of occupation and the weight of the mold is almost determined by the weight of these surface layers as observed when the mold has a complicated shape, when it is not possible to ensure a thickness sufficient for forming a two layer-structure, i.e., a surface layer.cndot.back-up layer structure because of restriction in shape and when the article to be molded has a two layer-structure. In these cases, the weight of the filter medium layer must accordingly be reduced, but it is impossible to reduce the weight thereof by the conventional techniques.